1. Field of the Invention
This invention relates to an optical semiconductor device, in particular, operative for high-speed modulation in response to an electric signal.
2. Description of the Prior Art
Research and develpment toward the realization of large capacities of trunk optical transmission systems are intensively being done. Also, under the current condition where optical fiber amplifiers have removed restriction on transmission distances caused by the optical loss of optical fibers, extension of transmission distance using an external modulation method with small wavelength chirping is desired. In particular, semiconductor optical modulators, which have small wavelength chirping even upon high-speed modulation and can be monolithically integrated with a semiconductor laser as a light source, are expected to be key devices supporting next-generation trunk optical transmission systems.
When a semiconductor optical modulator and a semiconductor laser are monolithically integrated, if there is any reflection at a facet of the optical modulator, reflected return light running into the laser region will results in inducing wavelength chirping. Therefore, the reflectivity of the emission facet of the modulator must be very small in a modulator/laser integrated light source. For example, the reflectivity of the facet must be as low as 0.01% or less, approximately, to ensure transmission of an optical signal at the rate of 2.5 Gbps over 500 km. For this purpose, mere coating of the emitting facet with a low reflective film is insufficient, and the use of a window-structure is indispensable.
FIG. 21 is a perspective view of the part of the emission facet of a conventional modulator/laser integrated light source combining a electro-absorption semiconductor optical modulator and a distributed feedback semiconductor laser. Numeral 1 refers to an n-type InP substrate, 2 to a light absorption layer, 3 to a Fe-doped semi-insulating InP buried layer, 4 to a p-type InP cladding layer, 5 to a p-type InGaAs contact layer, 6 to a p-type ohmic electrode made of Au/Zn/Au, 7 to a wiring and bonding pad made of Ti/Pt/Au, 8 to an n-type ohmic electrode made of AuGe/Ni/Au, 9 to a SiO.sub.2 film, and 10 to a low-reflective coating film of SiN.sub.x. In order to decrease the reflectivity of the emission facet, the light absorption layer 2 is partly removed near the emission facet to form a window region 15. Moreover, since high-speed modulation is not expected unless the capacitance of parasitic elements is low, a narrow mesa structure is used in the modulator region 16 and the window region 15.
In general, it is desirable that light output from a light source integrating an optical modulator and a semiconductor laser has an unimodal intensity distribution as far as possible to ensure a coupling efficiency with optical fibers.
FIG. 22 is a schematic explanatory diagram illustrating the light emitting from the light absorption layer of a conventional optical modulator/semiconductor laser integrated light source. In the modulator region 16, light propagates along the light absorption layer 2 also serving as a wave guide. In the window region 15 not having a guide structure, light is emitted in spread-out directions, and reflected or scattered by the side surface of the narrow mesa 14. A loss by scattering results in a decrease of optical output, and interference of reflected light largely distorts the intensity distribution of the emitted light. Therefore, the coupling efficiency of the conventional device with optical fibers was as low as 25%, and high-output devices could not be expected.
That is, if both a narrow-mesa structure and a window structure are used in conventional optical modulator/semiconductor laser integrated light sources, then an increase in loss in the window region and a decrease in coupling efficiency with optical fibers prevent an increase in optical output.